This invention is directed generally to a novel and improved thermal overload apparatus, and more particularly to apparatus for shorting an electrical line to ground in response to a thermal overload condition.
While the invention may find other applications, the description will be facilitated by particular reference to the use of a thermal overload apparatus for creating a short circuit to ground in response to a thermal overload of a surge arrester device. Surge arresters are commonly used in communications equipment, and in particular in telecommunications equipment, for shorting overvoltage conditions to ground, to protect the communications equipment. Typically, such surge arrester devices are coupled between a line, running to the equipment to be protected, and ground. In normal operation, the voltage and current on the line will reach the equipment unaffected by the surge arrester device. However, upon the occurrence of an overvoltage of a preselected magnitude, the surge arrester device will short the overvoltage to ground, preventing it from reaching the equipment.
Such surge arresters may take various forms. In the telecommunications field, so-called gas tube arresters have frequently been utilized. These gas tubes include a sealed tube or canister having a pair of electrodes separated by an inert gas-filled arc gap. When the voltage or electrical potential across these electrodes reaches a preselected level, an arc will form, thereby shorting the electrical potential across the electrodes. Normally, one electrode is coupled to the line to be protected and the other electrode to ground. The voltage level at which arcing occurs can be selected by selection of the width of the arc gap between electrodes as well as by selection of the inert gas filling the arc gap.
Similar arc gap protectors are also provided in the form of a pair of carbon electrodes separated by an air gap, which does not require a sealed container. Similar considerations apply, with the width of the arc gap across the carbon electrodes determining the arcing or breakdown voltage of the device.
More recently, solid state (thyristor or diode) devices have also been utilized in surge arrester applications, taking advantage of the reverse voltage breakdown properties of such devices. That is, a solid-state device is coupled between a line to be protected and ground, such that an overvoltage condition on the line meeting or exceeding the reverse breakdown voltage of the solid state device will be shorted to ground.
In some applications, additional backup devices or mechanisms have been provided. For example, in the case of gas tubes, often a secondary air gap has been provided. Usually such a secondary air gap is provided by using an additional conductive container to house the gas tube, and shaping this container so as to define an air gap between an inner wall of a the conductive container and one of the electrodes of the gas tube. Typically, such an air gap is sized so as to have an arcing voltage somewhat higher than the arcing voltage across the gas tube, in order to act as a backup device in the event of failure of the gas tube, for example by loss of the inert gas in the arc gap within the gas tube itself.
In addition to the foregoing, such protectors have sometimes employed various overcurrent protection arrangements in order to shunt across the surge arrester, in the event of prolonged overcurrent conditions or the like. One device often used in such an overcurrent protection arrangement in the telecommunications field is a so-called "heat coil." The heat coil device normally utilizes a wire-wound bobbin to melt a fusible or meltable link between the bobbin and a rod-like core running through the center of the bobbin. When the current through the wire-wound bobbin generates sufficient heat to melt this link, the movement of the bobbin and center rod element relative to one another will permit other elements of a related assembly to create a permanent short circuit to ground of the protector device. One example of a line protector having such a heat coil is shown in U.S. Pat. No. 3,849,750 which is owned by the same assignee as the present invention.
Some protectors have also employed some type of thermal overload or fail-safe mechanism to create a short circuit to ground in response to heating of the arrester caused by either a sustained overvoltage of a predetermined duration and level or a sudden large current surge. One type of thermal overload mechanism utilizes a fusible (i.e., meltable) element or pellet which is mounted in thermal contact with the surge arrester device. One of a number of arrangements of cooperating grounding elements is provided such that, upon the melting of the fusible element in response to a thermal overload condition, a spring will cause certain elements to shift or move to achieve shunting or shorting across the surge arrester. Examples of several different fail-safe arrangements of this type are shown in U.S. Pat. Nos. 4,314,302; 4,321,649 and 4,901,188 which are owned by the same assignee as the present invention.
While the foregoing prior art thermal overload arrangements have proven commercially successful, there remains room for further improvement. For example, the use of a meltable element as described above requires the correct assembly of the meltable element with the other cooperating parts of the protector assembly. Also, to restore service following activation of the foregoing types of fail-safe systems, such prior art devices require the entire protector assembly to be dismantled, the meltable element to be replaced, and the protector to be correctly re-assembled with a new meltable element.